This invention relates to weight training exercise apparatus, that is apparatus in which the user extends and contracts selected muscles or groups of muscles against the resistance of a weight.
In the early days of weight training, the apparatus used consisted of dumbbells and barbells. These articles suffered from inherent drawbacks, among them being instability, because they can be dropped, and awkwardness, because of the time necessary to change weights on them.
In the last few decades these drawbacks have been overcome by so-called weight stack exercise apparatus, in which a stack of incremental weights are guided for smooth vertical movement, a weight selector rod passes down through a plurality of vertically aligned bores and a selected weight is engaged with the rod by means of a pin passing through or under the weight and engaging in an aligned bore in the rod. In the exercise the whole stack of weights above the pin is lifted via the selector rod which is connected, for example, by a cable to hand grips or is directly connected, for example, to a lifting bar. In some arrangements there are more than one weight selector rod, and hereinafter the rod will be referred to broadly as "weight lifting means" to encompass the variety of arrangements possible.
The instability drawback is overcome by the guidance of the weights, and selection of different weights is made in seconds by withdrawing the selector pin from one weight and reinserting it in or against another. Weight stack exercise apparatus has therefore achieved widespread use, but although many drawbacks are overcome, there remain others which are inherent in the apparatus.
The main problem is the constraint which prevents the apparatus being useful to both light trainers and heavier trainers. Increments in weight are ideally small, say 5kg or 10kg. Thus ten weights would make up a 50 kg stack, but if say a 250kg maximum weight were wanted fifty weights would be necessary. Apart from the extra expense, there are inherent size limitations which prevent such a tall stack being viable. Conversely, to obtain a high maximum weight with a reasonable number of weights means that the increments, i.e. the sizes of each weight, are undesirably large. For this reason, commercial weight stack exercise apparatus has tended to be relatively lightweight, and users wanting high resistances have had to resort to barbells.
A second disadvantage, equally inherent in conventional barbells, is that the exercise resistance, ignoring friction or acceleration forces, is constant over the entire range of movement. Research has shown that this is not idea, but that for best results the resistance should vary over the range of movement. The devices which have been proposed in recent years to obtain improved contours of resistance to movement have involved complicated lever and/or cam mechanisms, which are expensive to manufacture and are prone to breakdown.